As is well known, a normal requirement for many types of burglar and fire alarm systems requires activation of an audible alarm to provide an indication of a monitored intrusion on the premises.
Many such systems serve a dual purpose to monitor both an unauthorized intrusion on the premises as well as monitoring or detecting a fire and to provide a suitable alarm for such undesired conditions.
In general, the systems employ a common alarm to indicate either a burglary, a fire or some other improper condition on the premises. In order to distinguish between an intrusion or another monitored condition, the common alarm or siren is activated by a different signal for a different condition. The user can then determine the condition by listening to the particular alarm.
One of the signals, for example, may be a steady tone which is generally used to denote a fire, while the other signal provides a wailing tone which may be indicative of an unauthorized intrusion such as the presence of a burglar.
A great many of such devices employ an audio amplifier and speaker arrangement to provide an audible signal indicative of the particular intrusion. A great many of these devices employ a high frequency oscillator which is shifted in frequency or modulated by a suitable low frequency to produce the audible or wailing frequency. The output of the same oscillator is used without the modulation or frequency shift to provide the steady tone signal indicative of a fire.
In any event, conventional ways of modulating or shifting certain high frequency oscillators serve to provide distortion which effects the duty cycle of the oscillator or the on/off ratio. Due to a change in the duty cycle, the audio amplifier circuits drive the speaker inefficiently and hence, the distortion results in providing a reduction in the effective output power in the loud speaker. The loud speaker and associated circuitry operate most efficiently with a symmetrical waveform applied thereto which is indicative of equal on and off time, each comprising for example, fifty percent of one cycle.
Thus, on most conventional alarm systems, the high frequency signal is normally between for example, 750 to 2,500 Hz. This signal is usually modulated at the rate of a few cycles per second to provide the wailing signal. During modulation, the oscillator loses symmetry as indicated above and hence, the audio circuit as well as the speaker operate at a reduced power level. It is, of course, understood that this result is undesireable as the lower power level results in a lower intensity audio alarm which therefore, has a reduced transmission range and hence, limits the area over which the alarm can be heard.
It is therefore an object of the present invention to provide an improved apparatus for driving a loud speaker alarm circuit to maintain an optimum power level, while further providing an economical and efficient way of producing a wailing tone.